How to update nested Records?

Not best practice: Create an opaque type instead.
Problem
Solution
Alternative Solution
Problem
type alias Movie = 
    { title : String
    , rating : Int
    }
  
type alias Model = 
    { currentMovie : Movie
    }
​
rate : Int -> Model -> Model
rate newRating model =
    { model 
    | currentMovie = 
        --{ model.currentMovie | rating = newRating }
        Debug.todo "the line above does not compile"
    }
Solution
type alias Movie = 
    { title : String
    , rating : Int
    }
  
type alias Model = 
    { currentMovie : Movie
    }
​
rate : Int -> Model -> Model
rate newRating ({currentMovie} as model) =
    { model 
    | currentMovie = 
        {currentMovie | rating = newRating }
    }
Alternative Solution
type alias Movie = 
    { title : String
    , rating : Int
    }
  
type alias Model = 
    { currentMovie : Movie
    }
​
rate : Int -> Model -> Model
rate rating ({currentMovie} as model) =
    model
    |> updateMovie (addRating rating)
​
updateMovie : (Movie -> Movie) -> Model -> Model
updateMovie fun ({currentMovie} as model) =
    { model | currentMovie = fun currentMovie}
   
addRating : Int -> Movie -> Movie
addRating rating movie =
    { movie | rating = rating}
This is the a better solution
Question
How can I update a nested record field?
Answer
First get the nested record, then update it:
let
    currentMovie = model.currentMovie
in
{ currentMovie | rating = newRating}
For function parameters we can use the keyword as to bind fields of a record to a variable with the same name:
rate newRating ({currentMovie} as model) =
    ...
Further reading
📄Article: Updating nested records in Elm by Wouter In t Velt
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Last updated 2 months ago